fix: resolve compiler warnings across codebase

- PaywallView: remove unnecessary nil coalescing for currencyCode
- GameDAGRouter: change var to let for immutable compositeKeys
- GamesHistoryRow/View: add missing wnba and nwsl switch cases
- VisitDetailView: fix unused variable in preview
- AchievementEngine: use convenience init to avoid default parameter warning
- ProgressCardGenerator: use method overload instead of default parameter
- StadiumProximityMatcher: extract constants to ProximityConstants enum

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

SportsTime/Features/Paywall/Views/PaywallView.swift

@@ -300,7 +300,7 @@ struct PricingOptionCard: View {
     private var pricePerMonth: String {
         if product.id.contains("annual") {
             let monthly = product.price / 12
-            return "\(monthly.formatted(.currency(code: product.priceFormatStyle.currencyCode ?? "USD")))/mo"
+            return "\(monthly.formatted(.currency(code: product.priceFormatStyle.currencyCode)))/mo"
         }
         return "per month"
     }